API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-129
At the time of API 581, Third Edition release, API Recommended Practice 941, Seventh Edition—Steels for
Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical
Plants—is being revised. Technology for investigating HTHA susceptibility and inspection methods for
detection and assessment of HTHA damage is being developed. The Third Edition of API 581 includes a
conservative screening criterion that allows the owner–user to flag components potentially affected by HTHA
(see Section 19.4) until a more quantitative risk assessment is developed based on a later edition of
API 941. Additionally, the most current edition of API 941 should be consulted for guidance on investigation,
inspection, and replacement.
This document does not:
a) prescribe changes in materials of construction for components that exceed limits defined in Section 19.4;
b) provide guidance for assessing HTHA damage.
This document provides a screening criteria to identify potentially susceptible components for a thorough
investigation. It is the owner–user’s responsibility to:
a) review, investigate, and determine the actual status regarding HTHA, including documenting the
procedures, assessment results, and conclusions;
b) conduct a thorough investigation and evaluate options for continued operation or replacement if HTHA is
detected in the component during an inspection.
19.4 Screening Criteria for Carbon and C-½ Mo and Cr-Mo Low Alloy Steels
If all of the following are true, then the component should be evaluated for susceptibility to HTHA.
a) The material is carbon steel, C-½ Mo, or a Cr-Mo low alloy steel (such as ½ Cr-½ Mo, 1 Cr-½ Mo, 1¼
Cr-½ Mo, 2¼ Cr-1 Mo, 3 Cr-1 Mo, 5 Cr-½ Mo, 7 Cr-1 Mo, and 9 Cr-1 Mo).
b) The operating temperature is greater than 177 °C (350 °F).
c) The operating hydrogen partial pressure is greater than 0.345 MPa (50 psia).
19.5 Required Data
The basic data required for analysis are provided in Table 4.1, and the specific data required for
determination of the DF for HTHA are provided in Table 19.1.
19.6 Determination of the Damage Factor
19.6.1 Overview
A flow chart of the steps required to determine the DF for HTHA is shown in Figure 19.2. The following
sections provide additional information and the calculation procedure.
19.6.2 Inspection Effectiveness
Currently there is no level of inspection effectiveness (LoIE) for HTHA damage.
19.6.3 Calculation of the Damage Factor
The following procedure may be used to determine the DF for HTHA; see Figure 19.2.
a) STEP 1—Determine the material of construction, exposure temperature, T, and the exposure hydrogen
partial pressure, P H2 .